Contao\CoreBundle\DataContainer\PaletteManipulator::applyFallbackPalette PHP Method

    private function applyFallbackPalette(array &$config, array $action)
    {
        end($config);
        $fallback = key($config);
        if (null !== $action['fallback']) {
            if ($this->canApplyToParent($config, $action, 'fallback', 'fallbackPosition')) {
                return;
            }
            // If the fallback palette was not found, create a new one
            $fallback = reset($action['fallback']);
            $this->applyLegend($config, ['name' => $fallback, 'position' => self::POSITION_APPEND, 'hide' => false]);
        }
        // If everything fails, add to the last legend
        $offset = self::POSITION_PREPEND === $action['fallbackPosition'] ? 0 : count($config[$fallback]['fields']);
        array_splice($config[$fallback]['fields'], $offset, 0, $action['fields']);
    }